Transmit-Receive Cross-modulation Distortion Analysis in Full Duplex Quadrature Balanced Wireless Transceivers

This paper presents an analysis of the nonlinear interaction between the transmit (TX) and receive (RX) signals in a quadrature balanced full duplex wireless transceiver system. A nonlinear analysis is conducted to describe the inband third-order distortion experienced by the RX signal resulting from the cross-modulation with the TX signal in a simultaneous transmit-receive scenario at the same frequency band. The theoretical analysis is verified in measurements of a quadrature balanced RF front-end, composed of two 5.5GHz class AB power amplifiers designed in 180nm CMOS process. The results indicate that the power ratio between RX carrier to third-order intermodulation distortion, corresponding to the RX EVM, is independent on the RX signal power level. At 10dB backoff from TX saturated power, RX carrier to third-order intermodulation distortion product is 34dBc, implying that at this TX power level and below no digital post-distortion operations are required to account for RX nonlinearities.